The present invention relates generally to refractive surgery, and in particular, provides a trephine and method which are well particularly well suited for separating a portion of a cornea along a lamella. Refractive surgery of a human eye may be used to treat myopia, hyperopia, astigmatism and certain corneal anomalies. Refractive surgery generally involves reshaping layers of the cornea to improve the refraction of light rays passing through the cornea. Refractive surgical methods include radial keratotomy, photorefractive keratectomy, and others.
The cornea, the transparent dome-shaped anterior portion of the eye, includes several distinct tissue layers. The anterior surface of the cornea is coated by a tear film, which covers the corneal epithelium. The protective epithelial tissue layer includes several layers of cells covering the front of the cornea. Below the epithelial layer is Bowman""s layer. Bowman""s layer is an acellular layer containing randomly oriented collagen fibers. Posterior to Bowman""s layer is the corneal stroma. Below the stromal layer is the membrane of Descemet. The posterior surface of the cornea is defined by the corneal endothelium, which is formed from a single layer of cells. The endothelial layer of cells covers the membrane of Descemet.
The corneal stroma is formed from about 60 connective tissue layers or xe2x80x9clamellaexe2x80x9d of crisscrossing collagen fibers. The lamellae are stacked over one another and substantially parallel to the corneal surfaces. In the average human eye the cornea is a part-spherical body about 0.5 mm thick over most of its extent; the stroma itself comprises about 90% of the total corneal thickness.
Known refractive surgical techniques often involve accessing the interior corneal tissue layers. For example, laser keratectomy (photo-ablative reprofiling of the cornea) is often performed by removing the central epithelium to expose the underlying tissue to a laser. Similarly, laser assisted in situ keratomileusis (LASIK) involves folding an excised flap of corneal epithelium and stroma laterally to gain access to the central stroma for laser sculpting. Accessing this internal tissue often involves forming precise cuts through the epithelium and into the stroma, often with specialized refractive surgical cutting devices called microkeratomes and trephines. Unfortunately, these known devices suffer from significant drawbacks. U.S. Pat. Nos. 4,180,075; 4,662,370; 5,133,726; 5,215,104; 5,290,301; 5,586,980; 5,624,456; 5,643,299; 5,653,723 and 5,690,657 relate to methods and apparatus for cutting ocular tissue and are herein incorporated by reference in their entirety.
Known trephines for ocular surgery are generally designed to cut the cornea across the corneal lamella. This transverse cutting has undesirable limitations. For one, it makes the depth of the incision difficult to control. For example, the sharpness of the surgical blade cutting the cornea can effect the cut depth. Also, because the cornea flexes during cutting, the depth of the incision may depend upon both the pressure and rate of cutting. Further, the orientation of the cutting across the lamella makes it difficult to separate the cornea along a lamella at the deepest portion of the incision.
Known micro-keratomes often make use of a vacuum ring and an applanation shoe within the vacuum ring. A knife blade makes a single guillotine like cut across the front of the cornea. The depth of the cut is determined by the height of the blade below the applanation shoe. In some procedures the cut-away portion is entirely detached from the remainder of the cornea. Generally, the cut is stopped such that the cut corneal tissue is not completely detached from the remaining cornea, but instead remains connected and forms a flap. This attachment allows the flap to be folded to one side, exposing the underlying stromal tissue for laser removal. The exposed stroma is sculpted with a laser. The flap is then folded back into its original position.
Recently, it has been proposed to use a high-speed water jet instead of a blade for lamellar keratotomy, in a technique termed hydro refractive keratoplasty (HRK).
Furthermore, while the depth of the guillotine like microkeratome blade may be controlled by applanating and cutting large areas of the cornea, this applanation may raise the intra-ocular pressure, thereby damaging the eye. Mechanical cutting across the front of the cornea may also prematurely fold the flap and produce irregular astigmatism. In surgical procedures to modify the shape of the eye, mechanical cutting of the central portion of the cornea may even produce an irregular surface. This irregular surface may form a scar and reduce visual acuity.
The present invention provides systems, methods and apparatus for precisely and controllably removing an anterior portion of an eye. In particular, the techniques of the present invention permit reversible removal of the front surface of a cornea along a lamella of the corneal stroma to permit refractive surgery. The present invention is particularly useful with refractive surgical procedures such as photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), or the like.
The methods and apparatus of the current invention permit less traumatic removal of a corneal flap for refractive surgery. The invention applanates a reduced area of the cornea by reshaping a border of a desired flap. For example, the applanation may occur along at least a portion of an annulus around a disc shaped area of the cornea. This applanation of a flap border (rather than the entire flap area) is less traumatic and reduces the resulting elevation in intra-ocular pressure.
Applanating an annular portion of the cornea flattens the corneal lamellae below the applanating surface. The flattened lamellae may then be cut with a blade, which is substantially parallel to the applanating surface. Because the lamellae are substantially parallel to the applanating surface, the blade is also substantially parallel to the lamellae. This parallel cutting helps to separate the cornea along a lamella at the depth of the incision. Since the blade is substantially parallel to the lamella, fluctuations in the distance the blade is inserted along the lamella produces little variation in the depth of the incision below the applanated surface. By controlling the separation between the blade and the applanating surface, the depth at which the lamellae separate may be precisely controlled. Therefore, by cutting the cornea with a blade that is parallel to a lamella at the maximum depth of the incision, the cornea may be cut very precisely to a predetermined depth and along a lamella.
The invention also provides for further separating the central cornea along a lamella without mechanically cutting the cornea with a sharp object such as a surgical blade. Because a border of the cornea has been cut along a lamella, the cornea may be further separated along the same lamella. For example, the operator may manually tease the lamellae apart, using gentle mechanical pulling to separate the flap from the underlying corneal tissues. Alternatively, a fluid may be injected along the lamella. If the fluid is injected as a jet, a lower pressure jet may be used for separating the stroma along a lamella than would be required for transverse cutting of the lamella. This method of separating is less likely to induce scarring.
From an apparatus aspect, the invention comprises a corneal trephine with a body having a reference surface for contacting and reshaping a border around an area of a cornea. The trephine further comprises a blade for cutting the reshaped cornea. The blade is rotatable around the body in alignment with and at a desired cut depth from the reference surface.
In one embodiment, the trephine further comprises a depth stop coupled to the body. The blade extends radially inward, and the depth stop limits the depth that the blade cuts into the cornea. This control of the cut depth into the cornea ensures that the deepest portion of the cut is under the reference surface.
In another embodiment, the trephine further comprises at least one tractional element. The tractional element releasably fixes the cornea to the reference surface. A tractional element will help maintain alignment of the reference surface with the cornea during cutting.
In a specific embodiment, the tractional elements comprise a plurality of vacuum holes. Vacuum holes are desirable tractional elements because they can releasably fix the cornea without disrupting the underlying surface. Also, the vacuum may be varied to control the fixation.
In another specific embodiment, the tractional elements comprise teeth. Teeth are desirable tractional elements because they can provide very strong fixation of the cornea, and are simple to construct.
In an additional embodiment, the trephine further comprises a rotational stop for limiting the rotation of the blade to a predetermined arc. This rotational stop provides precise control of the size of the region where the flap remains attached to the underlying tissue.
In an exemplary embodiment, the trephine comprises a ring having a flat annular surface for contacting and flattening an annular portion of a cornea. A blade for cutting the applanated cornea is supported relative to the ring and extends radially inward. The blade is rotatable along a plane substantially parallel to the surface so as to produce a cut which is substantially parallel to the flattened surface. A depth stop is coupled to the ring for controlling the depth the blade cuts into the cornea, and a rotational stop is coupled to the ring for stopping the rotation of the blade.
From a method standpoint, the invention comprises the steps of applanating a border around an area on a surface of a cornea, cutting the applanated cornea by inserting a blade into the cornea to a desired depth, guiding the inserted blade around the applanated cornea, and separating a flap of corneal tissue from a remaining corneal tissue at the cut depth.
The step of guiding may further include a step of stopping at a predetermined rotational position. This stopping creates an region where the flap is connected to the underlying tissue.
The step of separating may include separating the flap from the remaining tissue along a lamella without cutting a central portion of the cornea. This separating without cutting protects the central portion of the cornea from damage that can be caused by cutting.
The step of separating may further include a step of injecting a fluid between the lamella of the flap and remaining tissue. The injected fluid will facilitate separation along the lamella. Further, an injected fluid will cut the cornea more gently than a mechanical cut.
The injected fluid may comprise a jet. Forming a fluid into a jet provides additional control of the injected fluid for precise cutting. For example, the jet may be formed to a small diameter and directed along a lamella.
The injected fluid may be a gas or liquid. Gasses may be readily injected and are easy to pressurize. An injected liquid formed to a jet will maintain a narrow jet after leaving an orifice, thereby improving the precision of the cutting.
An alternate method of separating comprises teasing or gently pulling the flap from the underlying ocular tissue at the predetermined depth. This teasing is easy to perform and cost effective to implement.
By ablating an exposed surface of the separated cornea with a laser, a desired change in shape may be effected on the exposed surface. This ablating may be performed to correct vision. Also, the ablating is preferably performed with a pulsed excimer laser because of the smooth surfaces resulting from this type of laser ablation.
In light of the above, it is an object of the invention to cut a cornea by reshaping a surface comprising a border around an area of a cornea. It is a further object of the invention to cut the cornea in alignment with the reshaped surface. It is a yet further object of the invention to separate the cut cornea along a lamella.